On Tue, Jul 28, 2020 at 12:00:10PM -0400, Mathieu Desnoyers wrote:
> Add comments and memory barrier to kthread_use_mm and kthread_unuse_mm
> to allow the effect of membarrier(2) to apply to kthreads accessing
> user-space memory as well.
>
> Given that no prior kthread use this guarantee and that it only affects
> kthreads, adding this guarantee does not affect user-space ABI.
>
> Refine the check in membarrier_global_expedited to exclude runqueues
> running the idle thread rather than all kthreads from the IPI cpumask.
>
> This patch applies on top of this patch from Peter Zijlstra:
> "mm: fix kthread_use_mm() vs TLB invalidate" currently in Andrew
> Morton's tree.
>
> Signed-off-by: Mathieu Desnoyers <[email protected]>
> Cc: Peter Zijlstra (Intel) <[email protected]>
> Cc: Will Deacon <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
> Cc: Nicholas Piggin <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Cc: Andrew Morton <[email protected]>
> ---
> kernel/kthread.c | 19 +++++++++++++++++++
> kernel/sched/membarrier.c | 8 ++------
> 2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 48925b17920e..ef2435517f14 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1258,8 +1258,19 @@ void kthread_use_mm(struct mm_struct *mm)
> finish_arch_post_lock_switch();
> #endif
>
> + /*
> + * When a kthread starts operating on an address space, the loop
> + * in membarrier_{private,global}_expedited() may not observe
> + * that tsk->mm, and not issue an IPI. Membarrier requires a
> + * memory barrier after storing to tsk->mm, before accessing
> + * user-space memory. A full memory barrier for membarrier
> + * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
> + * mmdrop().
> + */
> if (active_mm != mm)
> mmdrop(active_mm);
> + else
> + smp_mb();
>
> to_kthread(tsk)->oldfs = get_fs();
> set_fs(USER_DS);
> @@ -1280,6 +1291,14 @@ void kthread_unuse_mm(struct mm_struct *mm)
> set_fs(to_kthread(tsk)->oldfs);
>
> task_lock(tsk);
> + /*
> + * When a kthread stops operating on an address space, the loop
> + * in membarrier_{private,global}_expedited() may not observe
> + * that tsk->mm, and not issue an IPI. Membarrier requires a
> + * memory barrier after accessing user-space memory, before
> + * clearing tsk->mm.
> + */
> + smp_mb();
> sync_mm_rss(mm);
> local_irq_disable();
Would it make sense to put the smp_mb() inside the IRQ disable region?
> tsk->mm = NULL;
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 168479a7d61b..8a294483074d 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -100,13 +100,9 @@ static int membarrier_global_expedited(void)
> MEMBARRIER_STATE_GLOBAL_EXPEDITED))
> continue;
>
> - /*
> - * Skip the CPU if it runs a kernel thread. The scheduler
> - * leaves the prior task mm in place as an optimization when
> - * scheduling a kthread.
> - */
> + /* Skip the CPU if it runs the idle thread. */
> p = rcu_dereference(cpu_rq(cpu)->curr);
> - if (p->flags & PF_KTHREAD)
> + if (is_idle_task(p))
> continue;
Do we want to add a:
WARN_ON_ONCE(current->mm);
in play_idle_precise() ?
Because, if I read this right, we rely on the idle thread not having an
mm.
